You can't deny something that's not proved yet, you see?
If there WAS a single documentary proof on such things like NKVD elimination teams directly behind the soviet forces, or soviets been burning civil buildings on retreat, there won't be any arguments.
Relics insist that's the story is authentic granting no proof. They don't even give away their true sources.
So all we do ask is either grant us the documentary proof on all the matters we're arguing about or make a 'fictional story disclaimer'. As we're aware that there is no proof to that we're cutting time to the disclaimer thing.
They don't need to prove anything. That's like asking George Lucas to prove that Hans Solo shot first, man. Its a work of fiction, meant for entertainment. It is inspired by real events, but at no point in the game (anywhere, afaik), do they state this is a documentary or do they try to "educate" the masses as to the accuracy of their story.
They said they were going for
authenticity: The uniforms, the geography, the vehicles and other assets are supposed to look similar to what they looked back then.
Even games that try to be very historical accurate, such as the Total War series, sometimes walk off the path of historical accuracy for the sake of gameplay and/or entertainment. That's why you can deploy ninjas in a battlefield, or deploy Katana Samurai Heroes, etc.
It's a game, not a documentary.